Page "Alpha decay" Paragraph 18

Being relatively heavy and positively charged, alpha particles tend to have a very short mean free path, and quickly lose kinetic energy within a short distance of their source.

This results in several MeV being deposited in a relatively small volume of material.

This increases the chance of cellular damage in cases of internal contamination.

In general, external alpha radiation is not harmful since alpha particles are effectively shielded by a few centimeters of air, a piece of paper, or the thin layer of dead skin cells.

Even touching an alpha source is usually not harmful, though many alpha sources also are accompanied by beta-emitting radio daughters, and alpha emission is also accompanied by gamma photon emission.

If substances emitting alpha particles are ingested, inhaled, injected or introduced through the skin, then it could result in a measurable dose.